1. Field of the Invention
The present invention relates to a method for manufacturing a glass substrate for a magnetic disc to be used as a recording medium of a computer or the like.
2. Description of the Related Art
In recent years, an information recording technique, especially a magnetic recording technique is remarkably progressing with advancement of an information technique. As substrates for magnetic recording media such as HDDs (hard disk drives) that is one of magnetic recording media, a glass substrate which can be made small and thin and made high in recording density as compared to an aluminum substrate which is used conventionally and which is excellent regarding the flatness and substrate strength of the surface of the substrate is adopted.
Additionally, in order to achieve high recording density in a magnetic disc to be used for HDDs or the like, a vertical magnetic recording type magnetic disc (vertical magnetic recording disk) has recently been suggested. With such an increase in information recording density, both linear recording density (BPI: Bit Per Inch) in a circumferential direction and track recording density (TPI: Track Per Inch) in a radial direction are being increased steadily. Moreover, a technique of narrowing the gap (magnetic spacing) between a magnetic layer of a magnetic disc and a recording/reproducing element of a magnetic head, thereby improving an S/N ratio, has also been studied. The floating height of a magnetic head which is desired in recent years is 8 nm to about 6 nm.
Moreover, high reading speed is required with the improvement in recording density as described above. In order to cope with this requirement, the rotational frequency at the time of reading tends to increase, and the rotational frequency of 7200 rpm or more is required in recent years.
In the above situations, as for chamfering of a glass substrate, a configuration in which end faces of the glass substrate are ground using a pulley-like rotating grindstone is also described conventionally in Patent Document 1 (Japanese Patent Laid-Open Publication No. 11-267975). In Patent Document 1, the entry angle of a grindstone with respect to the surface of the glass substrate is specified and two or more fine cooling water discharge holes are arranged in a grinding surface. Thereby, progress of chipping or cracking of end faces of the glass substrate is suppressed, so that the lifespan of a grindstone can be prolonged.
The reason why the chamfering is performed on the glass substrate is because generation of chips or foreign matters is prevented by making ends of the substrates smooth. If the surface state of an end face of the glass substrate is not smooth, there is a probability that resin or particles which are generated at this end face produce friction with a wall surface of a case made of resin, or other particles which are caught by an inner peripheral end face and an outer peripheral end face of the glass substrate may adhere to the main surfaces of the substrate in future. If a magnetic layer is formed while the particles have adhered to the main surfaces of the glass substrate, the particles cause film defects, or convex portions are formed in the surface of a magnetic disc. Thus, proper glide height is not obtained, and a thermal asperity failure is caused.
Additionally, as the ends are chipped, there is a probability that the strength of the glass substrate may deteriorate or the precision of concentricity may deteriorate. That is, the grinding of the ends of the substrate has been a big issue in order to reduce the rate of occurrence of chipping, improve production efficiency, and reduce manufacturing cost.
Meanwhile, polishing of main surfaces of a glass substrate is performed by providing a plurality of small holes in a carrier in which a sun gear and an internal gear meshes with each other so as to perform a planetary gear motion, and pressing polishing pads from above and below in a state where the glass substrate is mounted on each of the small holes (for example, Patent Document 2). Accordingly, two or more glass substrates can be polished at a time, and can be polished uniformly so that neither polishing traces nor variations may be generated in the substrates.    [Patent Document 1] Japanese Patent Application Laid-Open No. 11-267975    [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-280171
However, when end faces of a glass substrate is ground using a pulley-like rotating grindstone as described in Patent Document 1, irregularities (undulations) having a fixed interval of wavelength are formed at an outer peripheral end face. FIG. 6 is a view illustrating undulations formed at an outer peripheral end face of a conventional glass substrate. In order to facilitate understanding, the undulations are expressed in an emphasized manner in this drawing. As shown in this drawing, periodic undulations are formed at an outer peripheral end face 130 of a glass substrate 100. The undulations of this outer peripheral end face could be found by the visual inspection. However, since a magnetic head does not run on the outer peripheral end face and the outer peripheral end face was also mirror-polished, this was not conventionally acknowledged as a problem.
However, after the outer peripheral end face is polished, main surfaces are polished as described in Patent Document 2. At this time, since a glass substrate is mounted on each small hole, its outer peripheral end face rubs against a carrier. For this reason, if undulations are at the outer peripheral end face, more apexes of the undulations rub against an inner peripheral surface of the small hole of the carrier. Therefore, spotted scratches (abrasion) will be generated at the apexes.
Such damage causes generation of particles, and consequently, if the generated particles are attracted to the main surfaces, they cause convex defects after a magnetic disc is obtained, and cause a thermal asperity failure or a head crash. Additionally, metal ion included in the glass substrate is apt to be eluted from such damaged spots, and this causes corrosion which precipitates on the surface of the magnetic disc.
These problems are big problems with a further increase in capacity and a further increase in recording density in future. Additionally, it is known that, if grinding is performed with sufficient time, the undulations can be eliminated. However, since it is necessary to produce glass substrates on a large scale inexpensively, it is difficult to secure enough processing time to eliminate the undulations.